At present, developments have been being made in recordable optical disks having a high density, such as a DVD-R, DVD-RW, and DVD+RW, which have already been put into markets. As is well known, quality of recorded signals deteriorates unless the power of a laser beam (recording power) is at a proper value when those recordable optical disks are irradiated with a laser beam to perform recording. For an optical disk drive which records data on an optical disk, control of laser power used for recording becomes one of more essential techniques, along with the increasing density of the optical disk as a recording target.
In an optical disk drive, the laser power of the beam irradiated from an optical head varies due to a variety of factors. For example, even if the recording power is set to a proper value at the time of factory delivery, the power of the laser beam irradiated on an optical disk often takes an improper value when recording is actually carried out. Hence, an optical disk drive sets the recording power to a proper value via OPC control (Optimum Power Control) prior to recording, to improve recording quality.
In the OPC control, trial-writing is conducted by recording a signal having a predetermined pattern on a predetermined area of an optical disk while varying the laser power. Recording power is set on the basis of information obtained by reproducing the trial-writing signal. Known methods for the OPC control are a β method which uses a β value correlated with the asymmetry value of the reproduced signal, and a γ method which uses a γ value correlated with the signal amplitude of the reproduced signal. The recording power is set by use of either one of these methods. For example, Japanese Patent Application Laid-Open Publication Nos. 2001-283436 and 10-283656 describe methods in which an asymmetry value is obtained by use of the β method to set the recording power.
In general, the β method in which the asymmetry value is monitored based on the relationship between a long mark and a short mark is highly reliable in comparison with the γ method in which only the signal amplitude is monitored. When performing the OPC control for a phase-change DVD-RW or DVD+RW, however, the asymmetry value does not depend on the laser power in some media. That is, there is a case that the asymmetry value does not linearly change in relation to the laser power. In this case, the recording power cannot be set by use of the β method and thus the γ method is adopted.
Meanwhile, a phase-change optical disk has been known as causing a phenomenon (cross-erase) that a mark once recorded on a track adjacent to the track for recording thereon (recording track) disappears. This cross-erase tends to occur more frequently as the laser power increases. The present inventor has found a problem that if the recording power is set according to the γ method, cross-erase occurs in adjacent tracks although writing on a recording track may be achieved excellently. That is, selection of a recording power according to the γ method cannot optimize the recording power where consideration is taken into cross-erase in adjacent tracks.
When recording is performed on a phase-change optical disk, the recording power should desirably be determined in consideration of cross-erase. However, if recording is performed on both of a recording track and adjacent tracks and if a recording power is determined by inspecting influences of cross-erase, a drawback arises in that a longer time is required to select the recording power. No known conventional method has been capable of performing excellent writing on a recording track when performing recording on a phase-change optical disk together with capable of setting rapidly a recording power less influenced by cross-erase.